The optical measurement of the oxygen saturation of the haemoglobin in tissue supplied with blood has been known since 1932 and met with great interest at an early stage as a non-invasive method for the determination of the oxygen supply of living tissue in physiological research. Systems which were developed for the determination of this parameter originally made use of measurement methods which resembled spectrophotometric methods; in this case, the earlobe was frequently used as test object. In a similar way to a test cell, the earlobe was irradiated with light at different wavelengths. The oxygen saturation was computed from the measured intensities of the transmitted light. Principles of these computations were the known absorption spectra of haemoglobin in its form saturated with oxygen (oxyhaemoglobin, FIG. 1, spectrum 1) and its oxygen-free form (reduced haemoglobin, FIG. 1, spectrum 2). A difficulty in this method, which was designated as ear oxymetry, resided in the fact that, in contrast to a test cell, a precise calibration of the optical system was not possible and, accordingly, the measurement method remained relatively imprecise. It proved to be difficult to take adequate account of the influences of tissue thickness, blood content, light intensities and other variables. It was indeed inter alia attempted, by application of pressure to the tissue, temporarily to remove the blood contained therein and thereby to generate an artificial null point. However, experiments of this type proved to be impractical and too imprecise. In addition, use was made of the fact that the absorption coefficients of oxyhaemoglobin and reduced haemoglobin at a few wavelengths (e.g. 505 nm, 548 nm and 805 nm) have equal values (isoabsorption points 3, FIG. 1). By using one of these wavelengths, a reference point independent of the oxygen saturation was indeed established, but this reference point alone (without a second reference quantity) was not sufficient. Ear oxymetry was further refined in the sixties. Thus, in a system (HP 47201A ear oxymeter) developed by the company Hewlett Packard eight wavelengths were used, with the aid of which further reference points were used for the calibration of the measurement system. This method also proved to be very demanding and costly in implementation. Accordingly, ear oxymetry never gained access to clinical routine applications and was employed almost exclusively in physiological research and for specific experimental questions, e.g. aviation and other fields.
Not until the early eighties was a new method developed, which became known under the designation pulse oxymetry and which became well established within a short time as a routine method for monitoring patients, especially during anaesthesia and intensive care. The principle of pulse oxymetry is based on recording the changes in absorption in the test object, which changes are caused by the arterial blood flowing in in the form of pulses. This permits a measurement of the oxygen saturation of the haemoglobin in the arterial blood (arterial oxygen saturation). As shown in FIG. 2, the total absorption of light in tissue supplied with blood is composed of the following components:
Absorption 4 by tissue and bones PA1 Absorption 5 by venous blood PA1 Absorption 6 by arterial blood PA1 Variable absorption 7 as a consequence of volume changes which are generated by the pulsating arterial blood.
The first three components are, as a rule, steady over a relatively long period of time and are combined hereinbelow as the DC component. Only the fourth component (AC component) is periodically time-dependent in synchronism with the heart contractions.
As a rule, two light emitting diodes (LEDs) having wavelengths of approximately 660 nm (red) and approximately 890 nm (infrared) are used as light sources in pulse oxymetry. As is evident from FIG. 1, the light absorption of haemoglobin at 660 nm is greatly dependent upon the oxygen content of the blood. In contrast, the infrared wavelength is located in the vicinity of the isoabsorption point 3 of 805 nm, so that only a slight dependence of the light absorption upon the oxygen content is present. Accordingly, the absorption in the infrared range is used as reference quantity. The light emitted by the two LEDs is passed into a body part having a good supply of blood (e.g. pad of the finger, earlobe or toe). The light is repeatedly scattered therein and partially absorbed. The emerging light is measured by means of a photodiode which, as a rule, is disposed opposite to the LEDs. The two LEDs and the photodiode are usually integrated in one component, which is designated as a pulse oxymetric sensor. The separate measurement of the red and infrared light using only one photodiode is made possible by the use of alternating light pulses of the two wavelengths, which are metrologically detected and evaluated separately.